There is no doubt that the world needs a high brightness light emitting diodes (High Brightness Light-Emitting Diode; HB led), not only a high-brightness white light LED (HB WLED), comprising also a high luminance colored LED, to and from now on In the future, we will actively work with LEDs (Ultra High Brightness LEDs, referred to as UHD LEDs) that require ultra-high brightness.
Replacing the original EL backlight and CCFL backlight of the handheld device with LED backlight not only makes the circuit design simpler and easier, but also has higher external force resistance. Replacing the original CCFL backlight of LCD TVs with LED backlights is not only more environmentally friendly, but also displays more vivid and beautiful. LED lighting instead of white light, halogen light and other lighting, not only more bright and energy-saving, the use is also longer, and the lighting response is faster, can reduce the rear car collision rate when used in the vehicle lights.
Therefore, LEDs can only be used in the state of the electronic device in the past, to become the backlight of the liquid crystal display, and then extended to electronic lighting and public display, such as car lights, traffic lights, kanban message marquees, large-scale film and television Walls, even lighting in projectors, continue to extend their applications.
More importantly, the brightness efficiency of LEDs is the same as Moore'''s Law, which doubles every 24 months. In the past, white LEDs could only be used to replace incandescent and halogen lamps that consume too much power. That is, the luminous efficiency is within the level of 1030 lm/W. However, after the white LED breaks through 60 lm/W or even 100 lm/W, even fluorescent lamps and high-pressure gas discharge lamps are beginning to feel threatened.
Although LEDs continue to enhance brightness and luminous efficiency, in addition to the patented technologies such as the most fluorescent materials and mixed light, it will become an increasingly challenge for packaging, and it is a double challenge. On the one hand, packaging must be Let the LED have the maximum light extraction rate and the highest luminous flux to minimize the light loss, and also pay attention to the divergence angle of the light, the light uniformity, and the compatibility with the light guide plate.
On the other hand, the package must have the best heat dissipation of the LED, especially HB (high brightness) almost means HP (High Power, high power, high power), the current value of the LED is continuously increased, if not Good heat dissipation will not only reduce the brightness of the LED, but also shorten the life of the LED.
Therefore, the continuous pursuit of high-brightness LEDs, if the package technology used does not have the corresponding enhancement, then the high-brightness performance will be discounted. Therefore, this article will discuss more about the packaging technology of HB LEDs, including the discussion of optical communication. Also includes discussion on thermal conductivity.
Note 1: In general, HB LEDs refer to luminous efficiencies above 8 lm/W (8 lumens per watt).
Note 2: In general, HP LEDs use more than 1W (watts) of power, and the power consumption wattage is obtained by multiplying the forward voltage by the forward conduction current (Vf × If, f = forward).
Bare layer: "quantum well, multi-quantum well" enhances "light conversion efficiency"
Although this article is mainly about the enhancement of the luminous flux of the LED package, it is necessary to first explain the bare part of the deeper core. After all, the improvement of the bare crystal structure can also greatly increase the luminous flux.
The first is to enhance the efficiency of light conversion, which is also the most fundamental way. Only 15% and 20% of the electricity used per watt of LED is converted into light energy, and the rest are converted into heat energy and dissipated (waste heat). The key to improving this conversion efficiency is on the pn junction. The pn junction is the main illuminating heat-emitting position of the LED. The structural design change through the pn junction can improve the conversion efficiency.
In this regard, at present, Quantum Well (QW) is often built on the pn junction to increase the proportion of electricity converted into light energy, and further efforts will be made toward more cuttings. Multiple Quantum Well (MQW) technology.
Bare layer: "Refueling and remodeling, light and light folding". "LEDs with improved light output efficiency have already reached the public application. This is the traffic direction indicator on the periphery of the domestic construction site. LED group composition.
Note 3: AlGaInP (aluminum gallium indium phosphide) is also called "quaternary luminescent material", which is a combination of four elements of Al, Ga, In, and P.
Note 4: In the general graphic structure explanation, the pn junction is also called "lighting layer, emitting layer or active layer, active region".
Note 5: In addition to reducing light coverage and increasing reflection, sometimes the intention to switch to different technologies is to circumvent patents that have been applied for by other companies.
The luminous efficiency enhancement method of various AlGaInP LEDs, from left to right, the difference of technological advancement, the leftmost is the most basic standard LED geometry, and then begins to add DBR (Distributed Bragg Reflector) reflective layer, and then there is DBR and then The Current Blocking technology is added, and the OMA (Omni-directional Mirror Adherence) omnidirectional mirror bonding technology is used on the far right. This technology also changes the substrate material from GaAs to Si.
For LEDs of GaN and InGaN composite materials, there is also a set of process-structure optical flux enhancement method. In Germany, OSRAM is still using standard structure in 1999. In 2002, it progressed to ATON structure. In a better NOTA structure, in 2005 it was a ThinGaN structure.
Encapsulation layer: anti-aging yellow light, transmittance warfare
After trying to increase the brightness from the bare crystal level, it is officially taken over from the package level to ensure the highest light transmission and minimum light attenuation.
To have a high lumen retention (transmittance, transmittance, expressed in percentage units), the first step is the packaging material. In the past, the most common LED was epoxy, but after the epoxy resin aging It will gradually turn yellow (because of the "benzene ring" component), which will affect the bright color, especially the lower the wavelength, the faster the aging, especially some of the WLEDs use Near ultraviolet light, which has a lower wavelength than other visible light. , aging is faster.
The new proposal is to replace the epoxy resin with silicone. For example, the Luxeon series LED of Lumileds Company of the United States is a modified sealant.
It is not just Lumileds Luxeon that uses silicone, but other manufacturers also have silicone solutions, such as General Electric. Toshiba (GE Toshiba)'s InvisiSi1, Toray. Dow Coring Toray's SR 7010 and so on are also LED's silicone packaging solutions.
In addition to the low resistance to low wavelength, tannin extract is less susceptible to aging. It is also a kind of protection for human health, which is also a kind of protection for human health. In addition, the light transmittance, refractive index and resistance of silicone rubber The heat is ideal. GE Toshiba's InvisiSi1 has a refractive index of up to 1.51.53, a wavelength range of 95% between 350nm and 800nm, and 75% and 80% of light transmission at wavelengths as low as 300nm, or The refractive index is traded off to reduce the refractive index to 1.41, so that even a wavelength of 300 nm maintains a light transmittance of 95%. Similarly, Dow Coring Toray's SR 7010 has a light transmission of 99% above 405 nm and a refractive index of 1.51 after hardening. In addition, the heat resistance can reach 180 ° C and 200 ° C. I will not discuss it here.
In addition, some manufacturers have proposed a so-called resin-free package, that is, using glass as a jacket protection, or a ceramic package as proposed by Kyocera, Japan, for anti-aging, in which ceramics also have better heat resistance.
As the use time increases, the luminous flux of the LED will gradually decrease. In the figure, the luminous flux curve of the two LEDs is compared. The lower blue line is a general 5mm WLED indicator light, and the upper red line is a high power LED illumination.
Note 6: Another factor that accelerates the aging of epoxy resin is the temperature, which accelerates aging.
Encapsulation layer: reflection and refraction of the transflective cup of the lens
The aforementioned package is mainly to protect the LED bare crystal, and to protect the light and heat faithfully outward as much as possible after protection, and then at the package level, but no longer the inner Resin part, but the Lens part of the outer cover.
After being packaged with glue, there are various connection methods depending on the LED's different uses, such as making individual package components one by one. In the past, the most typical single LED indicator is the same, and the other is Multiple LEDs are combined into one integral component, such as a seven-segment display, a dot matrix display, and the like. In addition, there are two distinctions between soldering feet, namely, Through-Hole Technology (THT) and Surface-Mount Technology (SMT).
For the time being, I don't talk about clustered seven-segment displays and dot-matrix displays. In terms of independent, separate, and discrete packages, they also have different package appearances depending on different applications. If they are the same as the previous LEDs, The status indicator for the piercing welding is only a package of the Lamp type (also commonly referred to as the "bullet type" today), even if it is determined that this type also has the difference of the lens type (Lens Type). Such as typical Lamp, oval Oval, Super Oval Super Oval, Straight Flat and so on. If it is a surface-adhesive type, there are also Top View, Side View, Dome and so on.
Why do you have a variety of different lens shapes? In fact, there are also their own application requirements. In general, Lamp is used as the indicator number, Oval is used for outdoor signs or signs, Top View is used for direct-lit backlight, Flat and Side View with light guide (Guide Plate) :LGP) Do side-lit backlights, Dome as a small lighting bulb, compact flash, etc.
Different appearances and different applications, the viewing angle of the illumination is different. This part will test the package design again. Different design methods can be used to obtain different illumination angles, light intensities and luminous fluxes. There are four ways: Axial lens, flat lens, reflective cup, reflective cup, and reflective cup by island.
The general Lamp uses the central axis lens method, and the Dome and Oval/Super Oval are similar, but the Oval/Super Oval is more concentrated than the Lamp in the small axial direction. Flat uses the straight lens method. The advantage is that the optical angle of view is larger than that of the central lens method, but the disadvantage is that the luminous flux is reduced and the light intensity is weakened. For Top View, Side View, etc., a reflective cup or an island reflector cup is often used. This method is to add a mirror to the package to reflect and refract a part of the divergent angle beam, so that the angle and the light intensity can be balanced.
Nichia's 5mm white LEDs are shown in the Lamp-type package and are also designed with a bowl-shaped reflective cup to enhance the illumination angle and intensity.
As far as technical difficulty is concerned, the Axial lens and Flat lens using only the upper lens are relatively simple. As long as the transmission and beam divergence are considered, the opposite Reflective cup is different. The original transmission and divergence should be considered. Additional consideration of reflection, refraction, and beam convergence is indeed more complicated.
Also, we haven't discussed the material yet. In addition to the sustainable use of the original rubber material, the lens part can also be changed to other materials, because the lens has been more light-transparent and less concerned with bare crystal protection, so it can also adopt plastic. (Plastic), Acrylic, Glass, Polycarbonate, etc., and as mentioned before, the light transmittance is related to the wavelength, the wavelength of the different wavelengths is different, and there are different The material can be selected, even to color the lens, to increase the contrast of the light color, or depending on the decorative effect of the application (toy, Christmas tree), as well as the geometric design of the front lens, reflector cup, etc. The fourth issue on LED light.
Conclusion
Finally, HB LEDs have been emphasized as "green lighting". The implication is that "environmental protection" is a big point of appeal, so not only lead-free (Pb Free) packaging, but also today's European RoHS (Restriction of Hazardous Substances) Directive, the Restriction of Hazardous Substances Directive, does not contain mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB), polybrominated diphenyl Ether (PBDE), etc., regardless of package or LED. Environmental hazards, in addition to WEEE (Waste Electrical and Electronic Equipment directive) and other related regulations must also comply.
of course! We have also mentioned in the past that the package must be able to block and resist low wavelength, ultraviolet light, and have a certain hardness to resist mechanical external forces and heat resistance. In addition, insulation, antistatic and moisture resistance must also be note.
More importantly, whether you want high brightness or not, you must export the light as much as possible, because if you can't faithfully derive the light energy, the light energy will be absorbed in the encapsulation layer, which will be converted into heat energy, which is a heat dissipation problem on the package. Adding a concern factor, in fact, if the heat of the LED can not be smoothly solved and reduced, it becomes a heat load, and in turn, it will damage the LED body, including brightness, so that the best and most ideal light pass is achieved. It is a lesson that packaging design must pay attention to!
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